1. Field of the Invention
The present invention relates generally to vehicular control devices and methods and particularly to technological art employed to control an electric motor driven oil pump mounted in a vehicle having a mechanical oil pump driven by an engine to generate hydraulic pressure and a transmission hydraulically operated and receiving driving force from a rotating electric machine and transmitting the received driving force to a driving wheel.
2. Description of the Background Art
In recent years, as part of approaches for environmental issues, a hybrid vehicle is gaining attention that utilizes driving force received from a motor or a similar rotating electric machine to assist an engine, run the vehicle, and/or the like. One such hybrid vehicle receives driving force from a rotating electric machine and transmits the received force to a driving wheel via a transmission having a plurality of gears (or gear ratios). This transmission is implemented by that formed for example of a planetary gear. Such a transmission has a friction engagement element hydraulically operated to implement a gear as desired. This requires an oil pump for generating hydraulic pressure provided to the friction engagement element. As has been described above, a hybrid vehicle has a plurality of power sources such as an engine, a motor and the like. As such, it can use not only both of the power sources but also one of them to run. Accordingly, in addition to a mechanical oil pump driven by the engine, also providing an electric motor driven oil pump capable of generating hydraulic pressure even when the engine is stopped is considered as an approach to ensure hydraulic pressure while the engine is stopped.
Japanese Patent Laying-Open No. 2005-207304 discloses a control system provided for a hybrid vehicle having two oil pumps, i.e., a mechanical oil pump and an electric motor driven oil pump to control the oil pump without increased power loss or insufficient hydraulic pressure. The publication describes the control system for a hybrid vehicle in which a second prime mover (a rotating electric machine) is connected to an output member to which a power is transmitted from a first prime mover through a transmission in which a torque capacity is varied in accordance with an oil pressure, and which has a first hydraulic pump (a mechanical oil pump) driven by the first prime mover (an engine) for establishing an oil pressure to be fed to the transmission, and a second hydraulic pump (an electric motor driven oil pump) arranged in parallel with the first hydraulic pump and driven by an electric motor. The control system includes a torque limiting unit limiting an output torque of the second prime mover temporarily at a starting time of the first prime mover.
As the publication describes the control system for a hybrid vehicle, the second prime mover outputs a temporarily limited torque when the first prime mover starts. As such, if the first prime mover is not completely started and accordingly the second hydraulic pump is operated to generate hydraulic pressure, the torque input from the second prime mover to the transmission is limited, and the hydraulic pressure required or requested by the transmission is not particularly increased. Relative shortage of hydraulic pressure can thus be avoided.
When the engine is in operation and for example the driver turns off the ignition switch to stop the engine, the hydraulic pressure generated by the mechanical oil pump decreases. This reduces the torque capacity of the transmission connected to the rotating electric machine and there will not be transmission of torque between the rotating electric machine and the transmission. At the time, in stopping the engine, if the output shaft varies in how it rotates, the transmission connected via a drive shaft to the engine also internally experiences variation, e.g., has a planetary gear varied in how it rotates. This can abruptly close a gap (or a backlash) in the planetary gear. As a result in the transmission gears can collide against each other thus cause noise. This noise discomforts the driver and results in poor drivability. Furthermore when the engine is stopped and the output shaft has a number of revolutions per minute that passes through the engine's resonance frequency band, the output shaft significantly varies in how it rotates, and large vibration can occur. However, Japanese Patent Laying-Open No. 2005-207304 is silent on noise and vibration caused when an engine is stopped.